ALMA 200 pc imaging of a z~7 quasar reveals a compact, disk-like host galaxy

TL;DR

This study uses high-resolution ALMA imaging to reveal a compact, rotating, disk-like host galaxy of a z~7 quasar, showing intense star formation and a gas-rich core with minimal influence from the central black hole.

Contribution

First high-resolution imaging of a z~7 quasar host galaxy at 200 pc scale, revealing a disk-like structure with extreme star formation and detailed gas and dust properties.

Findings

The host galaxy is a compact, face-on, rotation-supported disk.

Central gas mass exceeds black hole mass, with no clear black hole influence on kinematics.

Star formation rate density exceeds 10^4 M_sun/yr/kpc^2.

Abstract

We present 0".035 resolution (~200 pc) imaging of the 158 um [CII] line and the underlying dust continuum of the z=6.9 quasar J234833.34-305410.0. The 18 h ALMA observations reveal extremely compact emission (diameter ~1 kpc) that is consistent with a simple, almost face-on, rotation-supported disk with a significant velocity dispersion of ~160 km/s. The gas mass in just the central 200 pc is ~4x10^9 M_sun, about a factor two higher than that of the central supermassive black hole. Consequently we do not resolve the black hole's sphere of influence, and find no kinematic signature of the central supermassive black hole. Kinematic modeling of the [CII] line shows that the dynamical mass at large radii is consistent with the gas mass, leaving little room for a significant mass contribution by stars and/or dark matter. The Toomre-Q parameter is less than unity throughout the disk, and thus is conducive to star formation, consistent with the high infrared luminosity of the system. The dust in the central region is optically thick, at a temperature >132 K. Using standard scaling relations of dust heating by star formation, this implies an unprecedented high star formation rate density of >10^4 M_sun / yr / kpc^2. Such a high number can still be explained with the Eddington limit for star formation under certain assumptions, but could also imply that the central supermassive black hole contributes to the heating of the dust in the central 110 pc.